The present invention relates to the production of images having image density variations, i.e., different gray levels. More particularly the invention relates to apparatus for producing representations of such gray levels in accordance with the halftone technique.
In traditional graphic arts work that technique has involved the photographing of a continuous tone original through a halftone screen. This produces a series of solid circular dots each having a diameter related to the average gray level of a corresponding picture element. In more recent times, halftone effects have been created by producing picture elements containing variably shaped dark areas created by any of a variety of different electronic processes. Typical examples are described in Behane et al. U.S. Pat. No. 3,604,846; Keller U.S. Pat. No. 3,922,484; and in Knox U.S. Pat. No. 4,246,614.
Another example of gray scale imaging is disclosed in Tschang U.S. Pat. No. 4,449,153 wherein a beam of light is directed through a light gate comprising an array of light valves. The ON time of each valve is selectively controlled in accordance with a desired gray level of a corresponding picture element. Light passing through the light valves is imaged onto an appropriate recording substrate. The Tschang patent also teaches that gray level additionally may be controlled by modulating the intensity of the light which is directed toward the light valves.